In general, while at present, there are a surface mount (square-shaped) electrochemical device, a coin-shaped electrochemical device, a thin electrochemical device and a cylinder-shaped electrochemical device as types of electrochemical device, all of the devices commonly have a structure in which a chargeable and dischargeable storage element and an electrolytic solution are sealed in a container and the storage element has a structure in which a separator is interposed between a positive electrode and a negative electrode. The separator is made of an ion-permeable porous sheet and functions to avoid a short circuit (short) of a positive electrode and a negative electrode and to deliver ions between the positive electrode and the negative electrode through the retained electrolytic solution.
When the separator delivers ions between the positive electrode and the negative electrode using the retained electrolytic solution, and the electrochemical device is repeatedly charged and discharged to be used for a long time, a phenomenon occurs in which the electrolytic solution retained in the separator decreases due to decomposition of the electrolytic solution and the like.
When the separator is made of a fiber sheet having approximately uniform porosity, and the electrolytic solution retained in the separator is decreased, a delivery path for ions is narrowly secured through the electrolytic solution which adheres to the surface of the fiber. However, since the fiber which forms the separator is not entirely provided parallel to a thickness direction of the separator, the delivery path for the ions is elongated in comparison with a case where a necessary and sufficient amount of the electrolytic solution is retained, a resistance value of the storage element is increased due to the elongated delivery path, and an inner resistance value of the electrochemical device is increased due to the increase in the resistance value, causing a drop in output voltage.
Even if the “approximately uniform porosity” of the separator is changed, it is difficult to avoid the increase in the resistance value of the storage element. That is, when the “approximately uniform porosity” is high, the delivery path for the ions in the separator may be easily cut. Meanwhile, when the “approximately uniform porosity” is low, a retainable amount of the electrolytic solution may be decreased so that a necessary and sufficient amount of the electrolytic solution may not be retained in the separator.
Patent Document 1 below describes a separator in which a high density layer of low porosity and a low density layer of high porosity overlap each other in a thickness direction of the separator. The respective high density layer and low density layer are not in contact with a positive electrode and a negative electrode, and therefore when the amount of the electrolytic solution retained in the separator is decreased, a delivery path for ions is elongated as described above, increasing a resistance value of a storage element.
In addition, Patent Document 2 below describes a separator in which the porosity of a portion protruding outwardly from a positive electrode and a negative electrode is lower than the porosity of a portion interposed between the positive electrode and the negative electrode. The porosity of the portion interposed between the positive electrode and the negative electrode is approximately uniform, and therefore when an amount of an electrolytic solution retained in the separator is decreased, a delivery path for ions is elongated as described above, increasing a resistance value of a storage element.